Powder coating compositions generally incorporate one or more film-forming ingredients and optionally one or more additional ingredients that enhance the manufacturability, use, and/or performance of the composition. Examples of such additional ingredients include colorants, fillers, slip additives, UV stabilizers, antioxidants, fluidizing agents, flow control agents, agents that modify surface tension, degassing agents, flexibilizing agents, coalescing agents, texturizing agents, antistatic agents, gloss modifying agents, luster agents (such as metal flakes), fungicides, bactericides, strengthening agents, toughening agents, biological agents, combinations of these, and the like.
Conventional processes for making powder coating compositions involve intimately mixing the ingredients, e.g., in an extruder, after which the extrudate is comminuted (e.g., ground or otherwise micronized) to obtain a powder of the desired particle size. This process involves intimate mixing of all the ingredients, and the composition of the resultant particles is generally homogeneous. Several other routes that are described in the patent literature are alleged to duplicate the results from a melt mixing process such as an extrusion process.
Making homogeneous particles is not always desirable. If the content of some additives is too high, the surface flow characteristics of the resultant powder can be degraded. For some ingredients, intimate mixing may not be practical from a manufacturing perspective. For instance, some formulations may be susceptible to segregation, leading to inconsistent performance. Mutually reactive ingredients may have a tendency to prematurely react under the temperatures and operating conditions typically involved during extrusion. Some ingredients are easily damaged, and thus not as compatible with extrusion processing as may be desired. Metal flakes, for example, can be badly crumpled during extrusion and/or grinding, causing the flakes to lose their luster.
The industry is already aware of these drawbacks. Consequently, mixtures of different particles have been used in some instances where use of a single particle is not practical. See, e.g., U.S. Pat. No. 4,260,066; EP 0 250 183 A; EP 0 372 958 A; and EP 0 389 080 A. Unfortunately, merely mixing particles has serious drawbacks. Simple dry blends may experience compositional shifts due to segregation that may occur during storage, transport, and/or use. Dry blends may also experience compositional shifts as they are reclaimed. Reclaim performance is important, because powder coating applications rarely if ever transfer 100% of the material to a substrate. Typical efficiencies are in the 60% to 70% range (although this is heavily dependent on the application and the substrate being used). To avoid the loss of a substantial portion of the powder, it is critical that material can be reclaimed. To assure that reclaimed material performs the same as the original material, the material must be compositionally stable over time.
To avoid the drawbacks of merely blending different particles, the industry has attempted to create fused agglomerates of different particles. With respect to color mixing, the GB 2226824 A patent application publication describes a powder coating composition of composite particles in which differently-colored particles are bonded together into composite clusters having a raspberry-like structure. This “raspberry” approach to forming agglomerates of particles also has been described in the powder coating literature to some degree in connection with metal additives. U.S. Pat. Nos. 5,856,378, 5,470,893 and 5,319,001 describe a similar approach. However, the methodology for making these agglomerates as described in these patents does not appear to be commercially viable. It is difficult to avoid equipment fouling and to control the particle size with this approach.
Consequently, although the concept of constructing composite particles from different particle components is appealing, a practical, effective way of doing this in a manner that has achieved widespread commercial acceptance has eluded the industry. Thus, there still remains a strong demand for some technical solution for how to effectively incorporate ingredients into compositionally stable powder coating compositions when all or some of the ingredients are not readily co-processable.
There has also been a strong demand for powder coating compositions that can be used to form coatings on temperature sensitive substrates such as plastics, wood, fabricated panels (e.g., particle board, medium density fibreboard, chipboard, plywood, paperboard, and the like), combinations of these, and the like. Application of powder coatings to temperature sensitive substrates such as these requires fundamental changes in the powder properties. Thermosetting materials generally undergo chemical curing reactions that are induced by heating, irradiation, and/or the like. The film-forming constituents typically are molten under the cure conditions to facilitate formation of uniform coatings. Since many of the substrates are heat sensitive, the ideal product would melt and cure at lower temperatures in comparison to conventional coatings applied to more heat-resistant substrates such as metal. This requires the use of catalysts, initiators, curatives, and/or reactants that would react under normal powder manufacturing processes (melt mixing in a single or twin-screw extruder) resulting in poor flow, gel formation, and inconsistent coating properties. In short, low temperature curing agents and systems are known, but these are difficult to compound into powder coating compositions inasmuch as the materials tend to react prematurely under the temperature conditions typically encountered when the powders are manufactured.
The research and development of commercially viable powder coating compositions that melt and cure at low temperatures thus has been extremely challenging. Some significant advances have been made, yet there is substantial room for more advancement. This is evidenced by the fact only a few low temperature powder coating compositions are in limited, commercial use in connection with coating temperature sensitive substrates. Research and development has been hampered in that traditional powder process technology is limiting in terms of formulation flexibility and process conditions.
So-called rapid cure powder coating systems have been suggested as one approach for coating substrates. Rapid-cure systems are of interest both for systems that are intended to be cured at higher temperatures (e.g., 300° F. or higher) as well as systems to be used at lower temperatures (e.g., below 300° F.). Systems that cure via irradiation (e.g., e-beam energy, ultraviolet energy, or the like) constitute illustrative examples of systems that cure rapidly, but thermally-induced rapid-cure systems also are known. However, rapid-cure systems are susceptible to chemical advancement, e.g., premature curing, not just during manufacture, but also during storage, handling, or use. Many rapid cure systems require refrigerated storage or have limited shelf life. It would be advantageous if the product could be shipped and stored under normal ambient conditions without risking chemical advancement. There is a need to find a way to impart adequate chemical stability to these systems so that they could be used more widely and more easily.
In addition, to obtain smooth coatings under low temperature and/or rapid cure conditions, it would be advantageous to use viscosity modifiers to obtain the desired flow characteristics. Many of the best flow modifiers, however, are crystalline in nature. These crystalline materials can be either reactive or non-reactive. Unfortunately, they are not as compatible and/or co-processable with other powder coating ingredients as might be desired. Melt mixing crystalline materials having low melt temperatures (i.e., melt temperatures below the Tg, i.e., the glass transition temperature, of the film forming resin(s), process temperature, and/or use temperature) with other powder coating ingredients often causes the mix to lose shear, causing the resulting product to be poorly dispersed. Further, to the extent that such materials might be co-processable, the amount that may be practically used is limited by compatibility issues. Ideally a production process could be uncovered that would allow inclusion of these crystalline raw materials in powder coating compositions at any desired level.
Crystalline raw materials with high melt points (e.g., those having melt points higher than the Tg of the film-forming resin(s), process temperature, and/or use temperature) can cause another type of problem in low temperature curing powder coatings. For instance, if a crystalline material has a melt temperature higher than the process temperature, it will not melt in the extruder. The extruder must then function as a grinder to break and distribute the material in the melt, which is not always desirable. This type of raw material is conventionally incorporated into a powder coating by using longer retention times in the extruder and/or using a more aggressive screw design. Since both changes cause higher process temperatures, they would not be acceptable for a powder designed to flow and cure at low temperatures. In addition, most of the base resins would be designed to be low in viscosity, greatly reducing the shear force available to break up chunks of crystalline material. Thus the resulting powder coating would not have a controlled domain size for the crystalline material, which would result in inconsistency in the final properties (both appearance and physical).
As another drawback, the addition of crystalline materials often leads to too low a Tg for the uncured powder. Liquid raw materials have a similar effect. For both cases, the low product Tg makes the product difficult to process in a traditional powder process since the blend does not solidify quickly on a chilled belt. Likewise, the product may not have returned to a glassy state on reaching the chipping or the grinding processes. Even if cryogenic conditions were used to overcome these in-process problems, the low product Tg would cause sintering and poor physical stability for the powder once it is placed in storage. Although small amounts of crystalline material can be incorporated without dropping the Tg below the industry standards (no sintering at 40° C. for 1 week), it would be much more desirable if a way could be found to use crystalline materials at much higher levels given the potential benefits if only the drawbacks could be reduced or avoided. Once again, it would be an advantage to have a process that can incorporate high levels of crystalline raw materials to meet the high flow and rapid cure requirements while still allowing shipment and storage under ambient conditions.
Many substrates of potential interest to the powder coating industry, e.g., wood, plywood, particle board, medium density fibreboard, and the like, often are only moderately conductive. Moisture is believed to be an important source of conductivity in such materials. Thus, regions that tend to dry out (sharp edges, corners, routed areas) are even less conductive. Consequently, it is difficult to consistently powder coat these substrates. Even if some areas are well covered, corners, routed areas and sharp edges remain a problem. Since powder charging is related to particle size distribution, it is critical that the particle size distribution (“PSD”) is tightly controlled. However, current grinding technology gives only limited control of the PSD. Additional control can be obtained by classification, but this raises issues of yield loss and the high probability that re-extruding fine particles, e.g., those having a particle size of less than about 10 μm, will lead to unacceptable chemical advancement for rapid cure formulations. To optimize the powder coating product, there is a need to obtain a controlled particle size distribution without feeding the fines back into the extrusion process.
For the same reasons it would also be an advantage to design a process that would allow the attachment of small amounts of materials designed to enhance or control charging effects.